In recent years, as the recording density of magnetic disk apparatus has increased, many magnetic heads utilizing the magnetoresistance effect (hereinafter such a head will be abbreviated as “MR head”) have been employed. In accordance with this trend, there is demand for a magnetic recording medium which has excellent magnetic characteristics, such as noise characteristics and coercive force.
This is because an MR head has high reproduction sensitivity and low head noise as compared with a conventional electromagnetic induction-type head, and thus a magnetic recording medium must have excellent magnetic characteristics, such as noise characteristics, in order to obtain enhanced characteristics of magnetic disk apparatus, such as S/N ratio and recording density.
At the present time, a generally-used magnetic recording medium comprises a substrate formed of an NiP-plated Al alloy, a non-magnetic undercoat film formed of Cr or a similar element which is formed on the substrate, and a magnetic film formed of a material predominantly comprising Co which is formed on the undercoat film.
A magnetic recording medium disclosed in Japanese Patent Publication (kokoku) No. 5-24564 is known as the aforementioned medium. The magnetic recording medium disclosed in this publication comprises a non-magnetic undercoat film which has a thickness of 50–200 Å and is formed of Cr, to thereby enhance squareness ratio.
Japanese Patent Application Laid-Open (kokai) No. 1-232522 discloses a magnetic recording medium which comprises a non-magnetic undercoat film comprising an alloy between Cr and one or more metals selected from among Cu, Nb, Ti, V, Zr, Mo, Zn, W, and Ta, the film having a thickness of 500–3000 Å, to thereby enhance magnetic characteristics, particularly coercive force.
However, in the magnetic recording medium disclosed in the aforementioned Japanese Patent Publication (kokoku) No.5-24564, the non-magnetic undercoat film formed of Cr is thin, and thus crystals of the material of the film grow insufficiently in the film. As a result, the magnetic film of the medium has poor crystal structure, thereby resulting in unsatisfactory magnetic characteristics, such as coercive force.
In the magnetic recording medium disclosed in Japanese Patent Application Laid-Open (kokai) No. 1-232522, a non-magnetic undercoat film formed of a Cr alloy is thick, and thus grains formed of a Cr alloy grow excessively in the film during film formation. As a result, when a magnetic film is formed on the non-magnetic undercoat film, the magnetic grains in the magnetic film increase in size and the grains grow epitaxially with respect to Cr alloy grains in the undercoat film. Therefore the magnetic recording medium provides poor noise characteristics.
A magnetic recording medium disclosed in Japanese Patent Application Laid-Open (kokai) No. 8-212532 has improved magnetic characteristics such as coercive force and noise characteristics. The magnetic recording medium disclosed in the publication comprises a magnetic layer of multi-layer structure in which many magnetic films and non-magnetic films are alternately stacked, and the magnetic film is formed of materials of high coercive force, such as a CoNiCrTa-type material, to thereby impart high coercive force and low noise to the medium.
Formation of a magnetic film or non-magnetic undercoat film in a magnetic recording medium is usually carried out by means of sputtering. Apparatus for carrying out sputtering are usually categorized into one of two modes, which are called a static counter film-formation mode and a motion counter film-formation mode.
A static counter film-formation mode refers to a mode in which, for example, a sputtering apparatus provided with a chamber comprising gate valves at either end of the chamber is employed; a target comprising a material for a film to be formed is placed in the chamber; a disk to be subjected to sputtering is conveyed into the chamber through a gate valve at one end of the chamber; the disk is halted at a position where the disk opposes the target; the disk is subjected to sputtering by use of the target in a static state; and after completion of sputtering, the disk is transferred to the other end of the chamber and removed from the chamber. In contrast, a motion counter film-formation mode refers to a mode in which a disk is subjected to sputtering in a chamber while the disk is conveyed from one end of the chamber to the other end without the disk being allowed to stop.
In each of these modes, a disk is subjected to sputtering while being conveyed in one direction, and carrying out film-formation twice or more times in one disk by use of a single target is not considered. Therefore, in order to form a layer comprising a plurality of films which are formed of the same material, as in the case of the aforementioned magnetic layer of multi-layer structure, another method; for example, one of the following three methods, must be carried out.
(1) After a film is formed on a disk in a first chamber by use of a target, the disk is transferred to a second chamber, and another film is formed on the disk by use of a target in the second chamber. Thereafter, the disk is transferred back to the first chamber, and the disk is again subjected to sputtering.
(2) After a disk is subjected to sputtering in a chamber, the disk is removed from the chamber and thereafter conveyed into the chamber again, to thereby again subject the disk to sputtering.
(3) A sputtering apparatus comprising a plurality of chambers comprising targets formed of the same material is employed, and a disk is conveyed into these chambers sequentially, thereby subjecting the disk to sputtering.
When the aforementioned methods (1) and (2) are carried out, productivity may decrease. Meanwhile, when the aforementioned method (3) is carried out, particularly, in order to form multiple films of the same material, a sputtering apparatus comprising chambers which match the number of the films must be employed, and thus the apparatus is required to be large in size, resulting in disadvantageously high production costs.
Generally, in order to form a magnetic film or non-magnetic film comprising a plurality of materials, the following methods, for example, are carried out.
(a) A film to be formed is subjected to sputtering by use of a target comprising an alloy of the same composition as the film.
(b) A portion of a plurality of materials for forming the aforementioned film is molded into chips and the chips are buried into another portion of the materials, or the materials are ground so as to obtain particles and the particles are mixed uniformly and aggregated, to thereby form a target of the composite material. The film is then formed through sputtering by use of this target.
(c) A plurality of targets which are formed of a portion of the aforementioned materials and differ from one another are prepared, and the targets are placed in one chamber of a sputtering apparatus. A film is formed through sputtering by simultaneous use of the targets; i.e., through co-sputtering.
However, when the above method (a) employing an alloy target is carried out, the mechanical strength of the target may be lowered, depending on the physical properties of the alloy, causing difficulty in production of a magnetic recording medium.
For example, in the case in which the material of a film to be formed comprises pure metals X and Y, which rarely form a solid solution region and are represented by a eutectic-type phase diagram, the pure metals X and Y coexist in the target in a small particle state when these metals are mixed to form a target, since these metals rarely form a solid solution. The bonding between particles comprising these pure metals X and Y is very weak, and thus the target has low mechanical strength and tends to break. Therefore, when such a target is employed, the target is easily broken during production thereof or sputtering, which may result in difficulty in production of a magnetic recording medium.
When the aforementioned materials differ significantly in specific gravity, a uniform target cannot be produced through a cast method usually employed, and thus a film of a desired composition may be difficult to form.
In the above method (b) employing a composite target comprising a plurality of materials, when the target is formed, impurities such as oxygen may enter interfaces between the materials. As a result, a film formed from the target will comprise large amounts of such impurities, and the magnetic recording medium produced may have poor magnetic characteristics such as coercive force and squareness ratio.
It has been reported that the magnetic characteristics of a magnetic recording medium, such as coercive force, can be enhanced by lowering the concentration of oxygen as an impurity in a film in the medium. For example, WO 95/03603 discloses such a phenomenon in a magnetic recording medium. Therefore, there is demand for a method for easily producing a magnetic recording medium of excellent magnetic characteristics, in which the concentration of the aforementioned impurities is lowered.
Particularly, when a target is employed in which a portion of a plurality of materials is molded into chips and the chips are buried into another portion of the materials, a film formed from the target exhibits non-uniformity in plane, and thus the magnetic recording medium produced may have poor magnetic characteristics.
In the above method (c) in which sputtering is carried out by simultaneous use of a plurality of targets, the targets must be placed in a single chamber. Therefore, in most cases, a portion of the targets must be set at a position which is unsuitable for forming a film that exhibits uniformity in a surface direction of a disk. Thus, a film formed from the targets may lack uniformity in plane, resulting in poor magnetic characteristics of the produced magnetic recording medium.
In view of the foregoing, an object of the present invention is to provide a magnetic recording medium which has excellent magnetic characteristics such as noise characteristics and coercive force, as well as a method for producing the medium efficiently and easily.
In addition, at present, further enhancement of the recording density of a magnetic disk apparatus or the like apparatus is a serious issue. Under the circumstances, there is demand for further enhancement of magnetic characteristics of a conventional recording medium.
In view of the foregoing, an object of the present invention is to provide a magnetic recording medium having excellent noise characteristic and excellent magnetic characteristics such as coercive force.
Furthermore, when the conventional method (1) is employed, tiny amounts of impurities such as oxygen and nitrogen remaining in a chamber are deposited on the surface of a disk during transfer of the disk from one chamber to another chamber. Thus, a film formed through sputtering comprises considerable amounts of impurities, and a recording medium fabricated from the film may have poor magnetic characteristics.
In the conventional method (2), impurities comprised in a chamber migrate into the aforementioned film during transfer of the disk.
Similarly, when the conventional method (3) is employed, migration of impurities may cause degradation of magnetic characteristics. In addition, forming a plurality of layers comprising the same material requires a sputtering apparatus including a plurality of chambers corresponding to the number of layers. Thus, a large-scale apparatus is required, thereby disadvantageously elevating production cost.
These three methods require a long period of time for forming a film, and the production cost disadvantageously increases. The cumbersome operations of these methods are also unsatisfactory.
Therefore, there is demand for a method for forming a magnetic film which provides excellent magnetic characteristics at high efficiency and with ease.
In view of the foregoing, an object of the present invention is to provide a magnetic recording medium having excellent magnetic characteristics, such as coercive force, and an excellent noise characteristic. Another object of the invention is to provide a method for producing the magnetic recording medium at high efficiency and with ease.